1. Field of the Invention
The present invention relates to electronic circuits and, more specifically, to an oscillating circuit that employs a phase-locked loop.
2. Description of the Prior Art
A voltage controlled oscillator (VCO) is a circuit that generates a periodic waveform having a frequency that is a function of a voltage input. In many cases, a differential voltage is applied to the VCO, which generates a waveform having a frequency that is linearly proportional to the differential voltage.
Many phase-locked loops (PLLs) that are used as clocking circuits in integrated circuits employ VCOs to generate a stable periodic signal based on the differential voltage applied to it. A PLL is an electronic circuit that controls an oscillator so that it maintains a constant phase angle relative to a reference signal. PLLs are used in a variety of different types of circuits, including communication circuits and digital integrated circuits. Because the VCO is highly sensitive to noise in the differential voltage applied to it, such noise will manifest itself as a deviation from the desired frequency of the PLL. PLLs are extensively utilized as on-chip clock generators to synthesize and de-skew a higher internal frequency from the external lower frequency.
Digital integrated circuit implementations of PLLs frequently experience substrate coupling due to simultaneous circuit switching and power/ground (P/G) noise, which results in a timing jitter. In a PLL, random variation of the phase, or jitter, is a critical performance parameter. When used in a microprocessor (or other type of logic chip), PLL jitter can cause the effective cycle time of the microprocessor to be increased to meet the chip logic timing due to the necessity of avoiding incorrect cycle changes due to noise. This results in reduced yield and thus increases the cost to meet a desired cycle time. Power supply noise is one the main contributors to jitter in PLLs.
Therefore, there is a need for a PLL that minimizes jitter in the differential voltage applied to the VCO contained therein.